1. Field of the Invention
This invention generally relates to a communication system for the acquisition of data from a large number of spatially distributed locations, and more specifically to a tree type communications system utilizing a plurality of spatially distributed addressable multiplexing modules to access a specific register out of a large number of registers.
2. Background Art
Monitoring in agricultural contexts is becoming increasingly important for providing information about the status of stored-commodities and their environments. In an integrated pest management system, the use of chemical treatments and other methods of insect control are dictated by the early and localized detection of infestations. Because of the physical distances involved in agricultural contexts, monitoring often involves inspecting many locations distributed over large areas. Automated monitoring systems involving computer acquisition of data from sensors distributed throughout stored-commodities eliminate the need for scheduling costly manual inspections and permits access to real-time data from all storage regions. These data can alert personnel to the need for control measures and can be input directly to expert management decision support systems. When electronic sensing is used, the practicality of monitoring is dictated by the economics of implementation. Often the major cost is not for the remote sensor hardware but rather is for the means of getting the data back into a computer at a central location. The direct approach of an individual cable for each sensor is physically unwieldy and economically impractical when hundreds or thousands of sensors are involved and the computer is thousands of feet away.
For example, W. J. Eradus discloses in Intelligent Sensors and Agriculture an "Agrinet" system where a master-slave relationship is set up between a computer and a plurality of sensors. The computer can poll every sensor to respond with measured data through a bus. However, Agrinet relies on the use of "intelligent sensors." These sensors usually include microcomputers to provide multiplexing, signal processing and handling of communications. These "intelligent sensors" are relatively expensive due to their complexity and thus limits the amount of sensors feasible for any one system.
John E. Judd discloses in a Sensor Highway A New Cost Effective Approach to Multiple Point Data Collections for Plant Machinery, a "sensor highway" system for collecting data from multiple point data collection sites. A controller sends a proper address code down one of three available buses connecting and powering a chosen sensor. The operator may then collect the data of that sensor. The sensor outputs are wired to a transducer adapter module (or TAM) which monitors incoming coded address data, provides connection to the sensors and provides an isolated direct two wire line to pre-conditioned sensors. The TAM is connected to the main highway via a branch of the sensor highway via a T adaptor or a pressed-on insulation displacement connector. However, the sensors of the sensor highway are limited to 250 per line and require multiple buses, increasing the amount of wiring necessary for the system and increasing its complexity and cost.
W. G. Gensler discloses another typical measurement system in the agricultural environment An Electrochemical Water System. Electronic equipment in the field make a multiplexed measurement from a probe. The data is serialized and encoded before being transmitted by an antenna. Again, the complicated structure for the transmission of the data results in a costly system which limits the number of sensors economically feasible.